"I thought that a larger transformer would better regulate against line spikes and voltage fluctuations" Maybe, but not always. Keep in mind that low voltage often requires higher current (though LEDs are not as bad as incandescent bubs). Wiring that runs concealed inside walls get smacked by the whole electric code unless the voltage and current are both low (power limited circuits) Wiring on the underside of cabinets at low voltage gets a lot of breaks. Even more if the circuit is also low power (power limited circuits, think like your doorbell or thermostat). Even with LEDs you may not get all tat much light from theses low power circuits though. LEDs sound great at only 10 mA each, until you need a lot of them for bright task lighting, let alone if you want a 3-color setup for varying light color.

What ever you do, do not us irrigation wire nuts. Don't know where you are located but if you are near a large city or in one there should be a Ewing Irrigation Store close. Ewing1.com. They sell 3-M DBRY splice kits meant for higher amps than irrigation connectors. You have less than 50 watts on your 12/2 wire now and you are talking about adding another 15 watts for a total of less than 70 watts... ... ... not an issue what so ever unless you are planning on running those two lights over to your neighbors yard or you live on 10 or more acres and want to put them at the top of your driveway. Is your transformer multi-tap? If so put the 12/2 on the 15 volt tap that way you will have enough oomph to drive the last LED. The industry standard for 12 volt LEDs is an operating range of 9 to 15 volts. Less than 9 volts and the LED will not come on, more than 15 volts and you will burn out the circuit that drives the LED. As always take any advise with a grain of salt and read the instructions that came with the light fixtures and through the salt over your left shoulder. Hope that helps.

That forum seems to be mostly interior lighting question. I was hoping to get some feedback from landscape pros who have hands on experience with different manufacturers.

Hello Everyone. Have no fear, the LED strip lights I'm using only pull 5.5watts per foot. I'm doing about 15 feet of lights. 15x5.5= 82.5watts @ 12v DC. The longest run is probably 15 feet maybe? The wire could easily be 18 AWG or even 20 AWG in all seriousness (although wouldn't be recommended). 14 AWG is MORE than enough. :-) Thanks so much for the sanity check and verification that using the NM cable is all good in this use case. After writing this I did find other posts on other forums of people who had the same scenario. It seems to check out from multiple sources. Thank you.